In a flame of pure hydrogen gas, burning in air, the hydrogen (H) chemically combines with oxygen (O) to form water (H2O) plus a lot of heat is produced. It does not produce other chemical by-products. Hence a key feature of hydrogen as a fuel is that it is non-polluting (since water is not a pollutant). Pure hydrogen does not occur naturally; it takes energy to manufacture it. Once manufactured it is an energy carrier (i.e. a store for energy first generated by other means). The energy is eventually delivered as heat when the hydrogen is burned. The heat in a hydrogen flame is a radiant emission from the newly formed water molecules. The water molecules are in an excited state on initial formation and then transition to a ground state, and the transition unleashes thermal radiation. This heat can provide motive power for cars, boats and airplanes. Smaller devices can also be powered by hydrogen through the use of hydrogen fuel cell batteries, which can power an electric motor.
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